Indeed, at sea, the wave motion causes, among other things, heave, i.e. an oscillating motion of vertical translation of the flooring units. When the latter support tools such as drill bits, the heave needs to be compensated for so that the tool is permanently in contact with the hole bottom.
To compensate for such motions, there are three major families of devices:                devices that are set in the drill string,        devices that are interposed between the string and the drill rig lifting system, and        devices that are integrated in the lifting system.        
The device type of the third family conventionally solves the heave compensation problem by making a first block, referred to as crown block, mobile, and if also comprises a second block, referred to as travelling block. The second block is called travelling block because if is mobile relative to the mobile unit. However, through the agency of the heave compensator, the second block is made substantially stationary in relation to the sea bottom. It is reminded that a block is a mechanical device allowing a load to be lifted by several cable strands.
Furthermore, this type of device generally comprises at least one cylinder connected to accumulators, notably pneumatic accumulators. These accumulators occupy a large volume, which is penalizing, in particular for an offshore application.
Documents KR-2,012,035,432 and CN-101,654,145 describe examples of compensation systems.
An important quantity for such a device is the variation of the force exerted on the first block (crown block) according to the motion of the mobile unit in relation to a constant value the device should support without heave. The difference between the real force and this constant value is referred to as error.
Furthermore, document FR-2,575,452 (U.S. Pat. No. 5,520,369) describes such a system comprising two blocks, at least one compensation cylinder connected to accumulators, a cable and two articulated arms including sheaves and rods that allow to compensate for a motion for an element hanging from a mobile unit. This system allows to reduce the volume of the accumulators through a suitable geometry of the articulated arms and to partly reduce the error. However, for this design, the volume of the accumulators remains large (approximately 16 m3 for a conventional design), and compensation errors remain significant; the force on the load is hardly constant.
The present invention relates to a motion (heave) compensation system for a load hanging from a mobile unit, the system comprising two blocks and hybrid damping means. According to the invention, the damping means comprise an oleopneumatic damping system and an electric drive system. The combined use of an electric drive system and of an oleopneumatic damping system allows to reduce or even to cancel the compensation error while decreasing the accumulation volume required for the oleopneumatic damping system. Furthermore, unlike entirely passive approaches, the damping means of the system according to the invention allow to compensate for certain nonlinearities, and therefore to reach better performances.